Bacteriophage T7 is a relatively simple virus that has been well characterized physiologically and genetically. Mutations are now available in over 30 T7 genes, affecting almost all of the proteins specified by T7 DNA. Approximately 10 genes involved in replication and maturation of T7 DNA have been identified, and mutant strains of Escherichia coli that do not permit normal replication of T7 DNA are also becoming available. Thus, the T7 - E. coli system is an excellent one in which to study the molecular details of DNA replication. Intermediates produced during replication and maturation of T7 DNA are being identified and isolated by sedimentation or gel electrophoresis of extracts of infected cells, and restriction endonucleases are being used to analyze their structures. Kinetics of movement of parental and progeny DNA through such forms are being followed for wild-type and mutant phages. Other aspects of DNA metabolism, such as protection of T7 DNA after infection, selective degradation of host DNA, initiation of replication, and mechanism of formation of deletions will also be studied. Fragments of T7 DNA are being cloned in plasmids of E. coli. Cloned fragments are able to recombine genetically with infecting T7 phages, and in some cases are able to provide T7 functions. These clones are being used to physically map T7 genes and genetic signals, for rapid genetic mapping, hybridization probes, etc. Study of interactions between cloned DNA and infecting phages should also provide information about DNA metabolism after infection.